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<h1>Deferred Reference</h1>

<p>This document is a guide to the behaviour of the <code
class="API">twisted.internet.defer.Deferred</code> object, and to various
ways you can use them when they are returned by functions.</p>

<p>This document assumes that you are familiar with the basic principle that
the Twisted framework is structured around: asynchronous, callback-based
programming, where instead of having blocking code in your program or using
threads to run blocking code, you have functions that return immediately and
then begin a callback chain when data is available.</p>

<p>See these documents for more information:</p>

<ul>
<li><a href="async.xhtml">Asynchronous Programming with Twisted</a></li>
</ul>

<p>
After reading this document, the reader should expect to be able to
deal with most simple APIs in Twisted and Twisted-using code that
return Deferreds.
</p>

<ul>
<li>what sorts of things you can do when you get a Deferred from a
function call; and</li>
<li>how you can write your code to robustly handle errors in Deferred
code.</li>
</ul>

<p>Unless you're already very familiar with asynchronous programming,
it's strongly recommended you read
the <a href="async.xhtml#deferreds">Deferreds section</a> of the
Asynchronous programming document to get an idea of why Deferreds
exist.
</p>


<h2>Callbacks</h2>

<p>A <code class="API">twisted.internet.defer.Deferred</code> is a promise that
a function will at some point have a result.  We can attach callback functions
to a Deferred, and once it gets a result these callbacks will be called. In
addition Deferreds allow the developer to register a callback for an error,
with the default behavior of logging the error.  The deferred mechanism 
standardizes the application programmer's interface with all sorts of 
blocking or delayed operations.</p>

<pre class="python">
from twisted.internet import reactor, defer

def getDummyData(x):
    """
    This function is a dummy which simulates a delayed result and
    returns a Deferred which will fire with that result. Don't try too
    hard to understand this.
    """
    d = defer.Deferred()
    # simulate a delayed result by asking the reactor to fire the
    # Deferred in 2 seconds time with the result x * 3
    reactor.callLater(2, d.callback, x * 3)
    return d

def printData(d):
    """
    Data handling function to be added as a callback: handles the
    data by printing the result
    """
    print d

d = getDummyData(3)
d.addCallback(printData)

# manually set up the end of the process by asking the reactor to
# stop itself in 4 seconds time
reactor.callLater(4, reactor.stop)
# start up the Twisted reactor (event loop handler) manually
reactor.run()
</pre>

<h3>Multiple callbacks</h3>

<p>Multiple callbacks can be added to a Deferred.  The first callback in the
Deferred's callback chain will be called with the result, the second with the
result of the first callback, and so on. Why do we need this?  Well, consider
a Deferred returned by twisted.enterprise.adbapi - the result of a SQL query.
A web widget might add a callback that converts this result into HTML, and
pass the Deferred onwards, where the callback will be used by twisted to
return the result to the HTTP client. The callback chain will be bypassed in
case of errors or exceptions.</p>

<pre class="python">
from twisted.internet import reactor, defer

class Getter:
    def gotResults(self, x):
        """
        The Deferred mechanism provides a mechanism to signal error
        conditions.  In this case, odd numbers are bad.

        This function demonstrates a more complex way of starting
        the callback chain by checking for expected results and
        choosing whether to fire the callback or errback chain
        """
        if x % 2 == 0:
            self.d.callback(x*3)
        else:
            self.d.errback(ValueError("You used an odd number!"))

    def _toHTML(self, r):
        """
        This function converts r to HTML.

        It is added to the callback chain by getDummyData in
        order to demonstrate how a callback passes its own result
        to the next callback
        """
        return "<p>Result: %s</p>" % r

    def getDummyData(self, x):
        """
        The Deferred mechanism allows for chained callbacks.
        In this example, the output of gotResults is first
        passed through _toHTML on its way to printData.

        Again this function is a dummy, simulating a delayed result
        using callLater, rather than using a real asynchronous
        setup.
        """
        self.d = defer.Deferred()
        # simulate a delayed result by asking the reactor to schedule
        # gotResults in 2 seconds time
        reactor.callLater(2, self.gotResults, x)
        self.d.addCallback(self._toHTML)
        return self.d

def printData(d):
    print d

def printError(failure):
    import sys
    sys.stderr.write(str(failure))

# this series of callbacks and errbacks will print an error message
g = Getter()
d = g.getDummyData(3)
d.addCallback(printData)
d.addErrback(printError)

# this series of callbacks and errbacks will print "<p>Result: 12</p>"
g = Getter()
d = g.getDummyData(4)
d.addCallback(printData)
d.addErrback(printError)

reactor.callLater(4, reactor.stop); reactor.run()
</pre>

<h3>Visual Explanation</h3>

<div hlint="off" align="center">
<img src="../img/deferred-attach.png" />
</div>

<ol>
  <li>Requesting method (data sink) requests data, gets
  Deferred object.</li>
  
  <li>Requesting method attaches callbacks to Deferred
  object.</li>
</ol>
<img hlint="off" src="../img/deferred-process.png" align="left" /> 

<ol>
  
  <li>When the result is ready, give it to the Deferred
  object. <code>.callback(result)</code> if the operation succeeded,
  <code>.errback(failure)</code> if it failed. Note that
  <code>failure</code> is typically an instance of a <code
  class="API">twisted.python.failure.Failure</code>
  instance.</li>
  
  <li>Deferred object triggers previously-added (call/err)back
  with the <code>result</code> or <code>failure</code>.
  Execution then follows the following rules, going down the
  chain of callbacks to be processed. 
  
  <ul>
    <li>Result of the callback is always passed as the first
    argument to the next callback, creating a chain of
    processors.</li>
    
    <li>If a callback raises an exception, switch to
    errback.</li>
    
    <li>An unhandled failure gets passed down the line of
    errbacks, this creating an asynchronous analog to a
    series to a series of <code>except:</code>
    statements.</li>
    
    <li>If an errback doesn't raise an exception or return a
    <code class="API">twisted.python.failure.Failure</code>
    instance, switch to callback.</li>
  </ul> </li>
</ol>
<br hlint="off" clear="all" />

<h2>Errbacks</h2>

<p>Deferred's error handling is modeled after Python's
exception handling. In the case that no errors occur, all the
callbacks run, one after the other, as described above.</p>

<p>If the errback is called instead of the callback (e.g.  because a DB query
raised an error), then a <code
class="API">twisted.python.failure.Failure</code> is passed into the first
errback (you can add multiple errbacks, just like with callbacks). You can
think of your errbacks as being like <code class="python">except</code> blocks
of ordinary Python code.</p>

<p>Unless you explicitly <code class="python">raise</code> an error in except
block, the <code class="python">Exception</code> is caught and stops
propagating, and normal execution continues. The same thing happens with
errbacks: unless you explicitly <code class="python">return</code> a <code
class="python">Failure</code> or (re-)raise an exception, the error stops
propagating, and normal callbacks continue executing from that point (using the
value returned from the errback). If the errback does returns a <code
class="python">Failure</code> or raise an exception, then that is passed to the
next errback, and so on.</p>

<p><em>Note:</em> If an errback doesn't return anything, then it effectively
returns <code class="python">None</code>, meaning that callbacks will continue
to be executed after this errback.  This may not be what you expect to happen,
so be careful. Make sure your errbacks return a <code
class="python">Failure</code> (probably the one that was passed to it), or a
meaningful return value for the next callback.</p>

<p>Also, <code class="API">twisted.python.failure.Failure</code> instances have
a useful method called trap, allowing you to effectively do the equivalent
of:</p>

<pre class="python">
try:
    # code that may throw an exception
    cookSpamAndEggs()
except (SpamException, EggException):
    # Handle SpamExceptions and EggExceptions
    ...
</pre>

<p>You do this by:</p>
<pre class="python">
def errorHandler(failure):
    failure.trap(SpamException, EggException)
    # Handle SpamExceptions and EggExceptions

d.addCallback(cookSpamAndEggs)
d.addErrback(errorHandler)
</pre>

<p>If none of arguments passed to <code class="python">failure.trap</code>
match the error encapsulated in that <code class="python">Failure</code>, then
it re-raises the error.</p>

<p>There's another potential <q>gotcha</q> here.  There's a
method <code class="API">twisted.internet.defer.Deferred.addCallbacks</code>
which is similar to, but not exactly the same as, <code
class="python">addCallback</code> followed by <code
class="python">addErrback</code>. In particular, consider these two cases:</p>

<pre class="python">
# Case 1
d = getDeferredFromSomewhere()
d.addCallback(callback1)       # A
d.addErrback(errback1)         # B
d.addCallback(callback2)       
d.addErrback(errback2)        

# Case 2
d = getDeferredFromSomewhere()
d.addCallbacks(callback1, errback1)  # C
d.addCallbacks(callback2, errback2)  
</pre>

<p>If an error occurs in <code class="python">callback1</code>, then for Case 1
<code class="python">errback1</code> will be called with the failure. For Case
2, <code class="python">errback2</code> will be called. Be careful with your
callbacks and errbacks.</p>

<p>What this means in a practical sense is in Case 1, &quot;A&quot; will
handle a success condition from <code>getDeferredFromSomewhere</code>, and
&quot;B&quot; will handle any errors that occur <em>from either the upstream
source, or that occur in 'A'</em>.  In Case 2, &quot;C&quot;'s errback1
<em>will only handle an error condition raised by
<code>getDeferredFromSomewhere</code></em>, it will not do any handling of
errors raised in callback1.</p>


<h3>Unhandled Errors</h3>

<p>If a Deferred is garbage-collected with an unhandled error (i.e. it would
call the next errback if there was one), then Twisted will write the error's
traceback to the log file.  This means that you can typically get away with not
adding errbacks and still get errors logged.  Be careful though; if you keep a
reference to the Deferred around, preventing it from being garbage-collected,
then you may never see the error (and your callbacks will mysteriously seem to
have never been called).  If unsure, you should explicitly add an errback after
your callbacks, even if all you do is:</p>

<pre class="python">
# Make sure errors get logged
from twisted.python import log
d.addErrback(log.err)
</pre>

<h2>Handling either synchronous or asynchronous results</h2>
<p>
In some applications, there are functions that might be either asynchronous or
synchronous. For example, a user authentication function might be able to
check in memory whether a user is authenticated, allowing the authentication
function to return an immediate result, or it may need to wait on
network data, in which case it should return a Deferred to be fired
when that data arrives. However, a function that wants to check if a user is
authenticated will then need to accept both immediate results <em> and</em>
Deferreds.
</p>

<p>
In this example, the library function <code>authenticateUser</code> uses the
application function <code>isValidUser</code> to authenticate a user:
</p>

<pre class="python">
def authenticateUser(isValidUser, user):
    if isValidUser(user):
        print "User is authenticated"
    else:
        print "User is not authenticated"
</pre>

<p>
However, it assumes that <code>isValidUser</code> returns immediately,
whereas <code>isValidUser</code> may actually authenticate the user
asynchronously and return a Deferred. It is possible to adapt this
trivial user authentication code to accept either a
synchronous <code>isValidUser</code> or an
asynchronous <code>isValidUser</code>, allowing the library to handle
either type of function. It is, however, also possible to adapt
synchronous functions to return Deferreds. This section describes both
alternatives: handling functions that might be synchronous or
asynchronous in the library function (<code>authenticateUser</code>)
or in the application code.
</p>

<h3>Handling possible Deferreds in the library code</h3>

<p>
Here is an example of a synchronous user authentication function that might be
passed to <code>authenticateUser</code>:
</p>

<a href="listings/deferred/synch-validation.py"
class="py-listing">synch-validation.py</a>

<p>
However, here's an <code>asynchronousIsValidUser</code> function that returns
a Deferred:
</p>

<pre class="python">
from twisted.internet import reactor

def asynchronousIsValidUser(d, user):
    d = Deferred()
    reactor.callLater(2, d.callback, user in ["Alice", "Angus", "Agnes"])
    return d
</pre>

<p> Our original implementation of <code>authenticateUser</code> expected
<code>isValidUser</code> to be synchronous, but now we need to change it to handle both
synchronous and asynchronous implementations of <code>isValidUser</code>. For this, we
use <code class="API" base="twisted.internet.defer">maybeDeferred</code> to
call <code>isValidUser</code>, ensuring that the result of <code>isValidUser</code> is a Deferred,
even if <code>isValidUser</code> is a synchronous function:
</p>

<pre class="python">
from twisted.internet import defer

def printResult(result):
    if result:
        print "User is authenticated"
    else:
        print "User is not authenticated"

def authenticateUser(isValidUser, user):
    d = defer.maybeDeferred(isValidUser, user)
    d.addCallback(printResult)
</pre>

<p>
Now <code>isValidUser</code> could be either <code>synchronousIsValidUser</code> or
<code>asynchronousIsValidUser</code>.
</p>

<p>It is also possible to modify <code>synchronousIsValidUser</code> to return
a Deferred, see <a href="gendefer.xhtml">Generating Deferreds</a> for more
information.</p>

<h2>DeferredList</h2>

<p>Sometimes you want to be notified after several different events have all
happened, rather than waiting for each one individually.  For example, you may
want to wait for all the connections in a list to close.  <code
class="API">twisted.internet.defer.DeferredList</code> is the way to do
this.</p>

<p>To create a DeferredList from multiple Deferreds, you simply pass a list of
the Deferreds you want it to wait for:</p>
<pre class="python">
# Creates a DeferredList
dl = defer.DeferredList([deferred1, deferred2, deferred3])
</pre>

<p>You can now treat the DeferredList like an ordinary Deferred; you can call
<code>addCallbacks</code> and so on.  The DeferredList will call its callback
when all the deferreds have completed.  The callback will be called with a list
of the results of the Deferreds it contains, like so:</p>

<pre class="python">
def printResult(result):
    print result
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
deferred3 = defer.Deferred()
dl = defer.DeferredList([deferred1, deferred2, deferred3])
dl.addCallback(printResult)
deferred1.callback('one')
deferred2.errback('bang!')
deferred3.callback('three')
# At this point, dl will fire its callback, printing:
#     [(1, 'one'), (0, 'bang!'), (1, 'three')]
# (note that defer.SUCCESS == 1, and defer.FAILURE == 0)
</pre>

<p>A standard DeferredList will never call errback.</p>

<div class="note">
<p>If you want to apply callbacks to the individual Deferreds that
go into the DeferredList, you should be careful about when those callbacks
are added. The act of adding a Deferred to a DeferredList inserts a callback
into that Deferred (when that callback is run, it checks to see if the
DeferredList has been completed yet). The important thing to remember is
that it is <em>this callback</em> which records the value that goes into the
result list handed to the DeferredList's callback.</p>

<!-- TODO: add picture here: three columns of callback chains, with a value
being snarfed out of the middle of each and handed off to the DeferredList
-->

<p>Therefore, if you add a callback to the Deferred <em>after</em> adding the
Deferred to the DeferredList, the value returned by that callback will not
be given to the DeferredList's callback.  To avoid confusion, we recommend not
adding callbacks to a Deferred once it has been used in a DeferredList.</p>
</div>

<pre class="python">
def printResult(result):
    print result
def addTen(result):
    return result + " ten"

# Deferred gets callback before DeferredList is created
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
deferred1.addCallback(addTen)
dl = defer.DeferredList([deferred1, deferred2])
dl.addCallback(printResult)
deferred1.callback("one") # fires addTen, checks DeferredList, stores "one ten"
deferred2.callback("two")
# At this point, dl will fire its callback, printing:
#     [(1, 'one ten'), (1, 'two')]

# Deferred gets callback after DeferredList is created
deferred1 = defer.Deferred()
deferred2 = defer.Deferred()
dl = defer.DeferredList([deferred1, deferred2])
deferred1.addCallback(addTen) # will run *after* DeferredList gets its value
dl.addCallback(printResult)
deferred1.callback("one") # checks DeferredList, stores "one", fires addTen
deferred2.callback("two")
# At this point, dl will fire its callback, printing:
#     [(1, 'one), (1, 'two')]
</pre>

<h3>Other behaviours</h3>

<p>DeferredList accepts three keyword arguments that modify its behaviour:
<code>fireOnOneCallback</code>, <code>fireOnOneErrback</code> and
<code>consumeErrors</code>.  If <code>fireOnOneCallback</code> is set, the
DeferredList will immediately call its callback as soon as any of its Deferreds
call their callback.  Similarly, <code>fireOnOneErrback</code> will call errback
as soon as any of the Deferreds call their errback.  Note that DeferredList is
still one-shot, like ordinary Deferreds, so after a callback or errback has been
called the DeferredList will do nothing further (it will just silently ignore
any other results from its Deferreds).</p>

<p>The <code>fireOnOneErrback</code> option is particularly useful when you
want to wait for all the results if everything succeeds, but also want to know
immediately if something fails.</p>

<p>The <code>consumeErrors</code> argument will stop the DeferredList from
propagating any errors along the callback chains of any Deferreds it contains
(usually creating a DeferredList has no effect on the results passed along the
callbacks and errbacks of their Deferreds).  Stopping errors at the DeferredList
with this option will prevent <q>Unhandled error in Deferred</q> warnings from
the Deferreds it contains without needing to add extra errbacks<span
class="footnote">Unless of course a later callback starts a fresh error &mdash;
but as we've already noted, adding callbacks to a Deferred after its used in a
DeferredList is confusing and usually avoided.</span>.</p>

<a name="class"></a>

<h2>Class Overview</h2>

<p>This is an overview API reference for Deferred from the point of using a
Deferred returned by a function. It is not meant to be a
substitute for the docstrings in the Deferred class, but can provide guidelines
for its use.</p>

<p>There is a parallel overview of functions used by the Deferred's
<em>creator</em> in <a href="gendefer.xhtml#class">Generating Deferreds</a>.</p>

<h3>Basic Callback Functions</h3>

<ul>
  <li>
  <code class="py-prototype">addCallbacks(self, callback[, errback, callbackArgs,
  callbackKeywords, errbackArgs, errbackKeywords])</code> 
  
  <p>This is the method you will use to interact
  with Deferred. It adds a pair of callbacks <q>parallel</q> to
  each other (see diagram above) in the list of callbacks
  made when the Deferred is called back to. The signature of
  a method added using addCallbacks should be
  <code>myMethod(result, *methodArgs,
  **methodKeywords)</code>. If your method is passed in the
  callback slot, for example, all arguments in the tuple
  <code>callbackArgs</code> will be passed as
  <code>*methodArgs</code> to your method.</p>
  
  <p>There are various convenience methods that are
  derivative of addCallbacks. I will not cover them in detail
  here, but it is important to know about them in order to
  create concise code.</p>
  
  <ul>
    <li>
    <code class="py-prototype">addCallback(callback, *callbackArgs,
    **callbackKeywords)</code> 
    
    <p>Adds your callback at the next point in the
    processing chain, while adding an errback that will
    re-raise its first argument, not affecting further
    processing in the error case.</p>

    <p>Note that, while addCallbacks (plural) requires the arguments to be
    passed in a tuple, addCallback (singular) takes all its remaining
    arguments as things to be passed to the callback function. The reason is
    obvious: addCallbacks (plural) cannot tell whether the arguments are
    meant for the callback or the errback, so they must be specifically
    marked by putting them into a tuple. addCallback (singular) knows that
    everything is destined to go to the callback, so it can use Python's
    <q>*</q> and <q>**</q> syntax to collect the remaining arguments.</p>
    
    </li>
    
    <li>
    <code class="py-prototype">addErrback(errback, *errbackArgs,
    **errbackKeywords)</code> 
    
    <p>Adds your errback at the next point in the
    processing chain, while adding a callback that will
    return its first argument, not affecting further
    processing in the success case.</p>
    </li>
    
    <li>
    <code class="py-prototype">addBoth(callbackOrErrback,
    *callbackOrErrbackArgs,
    **callbackOrErrbackKeywords)</code> 
    
    <p>This method adds the same callback into both sides
    of the processing chain at both points. Keep in mind
    that the type of the first argument is indeterminate if
    you use this method! Use it for <code>finally:</code>
    style blocks.</p>
    </li>
  </ul> </li>
  
</ul>



<h3>Chaining Deferreds</h3>

<p>If you need one Deferred to wait on another, all you need to do is return a
Deferred from a method added to addCallbacks.  Specifically, if you return
Deferred B from a method added to Deferred A using A.addCallbacks, Deferred A's
processing chain will stop until Deferred B's .callback() method is called; at
that point, the next callback in A will be passed the result of the last
callback in Deferred B's processing chain at the time.</p>

<p>If this seems confusing, don't worry about it right now -- when you run into
a situation where you need this behavior, you will probably recognize it
immediately and realize why this happens.  If you want to chain deferreds
manually, there is also a convenience method to help you.</p>

<ul>
  <li>
  <code class="py-prototype">chainDeferred(otherDeferred)</code> 
  
  <p>Add <code>otherDeferred</code> to the end of this
  Deferred's processing chain. When self.callback is called,
  the result of my processing chain up to this point will be
  passed to <code>otherDeferred.callback</code>. Further
  additions to my callback chain do not affect
  <code>otherDeferred</code></p>
  <p>This is the same as <code
  class="python">self.addCallbacks(otherDeferred.callback,
  otherDeferred.errback)</code></p>
  </li>
</ul>
    
<h2>See also</h2>

<ol>
<li><a href="gendefer.xhtml">Generating Deferreds</a>, an introduction to
writing asynchronous functions that return Deferreds.</li>
</ol>

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